The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Friction stir welding (“FSW”) is a solid-state welding process in which a rotating tool heats and intermixes two workpieces at a seam. More specifically, the rotating tool has a pin that is pressed into the seam as the tool rotates, producing frictional heat between the tool and the workpieces. Enough heat is generated such that regions of the workpieces plasticize. A shoulder of the FSW tool assists in causing the plasticized regions to intermix, thus joining (i.e., friction stir welding) the workpieces at the seam. The rotating tool travels along the entire length of the seam to form a weld joint line between the two workpieces.
FSW provides numerous advantages over other welding processes, in part, due to the fact that FSW occurs at much lower temperatures and without a filler material. Some of the advantages of FSW include: better mechanical properties at the weld; less porosity, shrinkage, and distortion; little or no toxic fume emissions; no consumable filler material; and ease of automation. Since its conception in 1991, FSW has been heavily researched and successfully applied to numerous industries in a wide variety of applications.
For some FSW applications, the FSW tool must be maintained at an optimum penetration depth during the FSW process in order to achieve optimum weld quality. Under penetration (also referred to as lack of penetration or “LOP”) can lead to non-welded portions along the seam, thus reducing the strength of the weld. Over penetration (e.g., penetrating beyond the optimum penetration depth) and full penetration (e.g., penetrating through the entire thickness of the workpieces) can also reduce the strength of the weld. Full penetration can even damage the FSW tool and the anvil that supports the backside of the workpieces. Variations in the thickness of the workpieces can make it difficult to maintain an optimal penetration depth during the FSW process.
In order to address the problems associated with full penetration, various references describe the use of a sacrificial material between the anvil and the workpieces. JP2007319931 to Jeong, for instance, describes a sacrificial metal foil that is inserted between the workpieces and the anvil. The disclosed foil can be steel, aluminum or copper, or a metal or ceramic sprayed coating. However, the foil runs the risk of failing to contain enough material to be effective when a friction stir welding pin deeply penetrates the members to be joined.
Of particular importance in the present application are FSW anvils that include a sacrificial material. As used herein, “sacrificial material” refers to a material that has material properties that have been chosen to allow the sacrificial material to form part of a FSW weldment without substantially degrading the quality of the weldment.
Sacrificial materials have been used in other aspects of FSW processes. For example, U.S. Pat. No. 6,607,119B2 to Engelhard describes a sacrificial element that is used to lead a friction pin out of a connection zone. U.S. Pat. No 8,220,694 to Nakagawa et al. teaches inserting a sacrificial material with a relatively low melting point between the workpieces to form part of the weld. However, those of ordinary skill in the art have failed to appreciate that sacrificial materials can be used in FSW anvils to address problems with full penetration.
Some references have opposed including materials other than the workpieces along a FSW weldment. For example, EP1796865 to Packer et al. describes coatings, such as TiN, TiCN, and others, that can be applied to an outer surface of an anvil (e.g., mandrel) to prevent diffusion welding between the mandrel shell and a pipe joint during FSW. (For a general description of a FSW process, see Friction Stir Welding and Processing, by R. S. Mishra et al, Materials Science and Engineering R 50 (2005) 1-78).
WO2012093680 to Chikuma et al. generally explains a process of friction stir welding a lid on a cylindrical pressure tank. The lid engages an opening of the tank to form a cylindrical butt joint on the outside of the tank. The lid also has a smaller diameter end portion that fits inside the tank. A FSW tool is used to form a weld along the butt joint. The FSW tool penetrates the butt joint at a depth that is sufficient to form a weld region between the smaller diameter end portion of the lid and the tank.
These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, there is still a need for a sacrificial material that can reduce friction stir welding defects along a continuous seam between two workpieces and partially support the structure of the two workpieces.